The invention relates to a device for optically scanning a surface over a given stroke in the X direction in a rectangular system of coordinates XYZ, which surface is substantially parallel to the X-Y plane, said device comprising a radiation source for supplying a radiation beam, an objective lens for focusing the radiation beam on the surface in the Z direction, said objective lens being mounted in an objective system comprising a permanent magnet, and a system of coils for electromagnetically supporting and driving the objective system, said system of coils comprising a plurality of coil pairs, each coil pair comprising two coils arranged one above the other in the Z direction.
A device of this type is suitable, inter alia, for reading disc-shaped optical record carriers in which the information is arranged in a plurality of concentrical tracks or quasi-concentrical tracks together forming a spiral track. A single track on the record carrier can be scanned by rotating the record carrier. To scan other tracks, the objective lens must be moved across the tracks in the radial or X direction. For a short access time to the stored information, it must be possible to move the objective lens rapidly across the tracks.
A device as described in the opening paragraph is known from the article "A floating-lens actuator", published in Japanese Journal of Applied Physics, vol. 26 (1987) supplement 26-4, pp. 195-197. In this known device the objective system, with the optical axis along the Z axis again, comprises a ring magnet which is surrounded by a system of coils for electromagnetically supporting and driving the objective system in five degrees of freedom. The system of coils comprises three coil pairs which are circularly positioned around the objective system. By correctly driving the coils, the objective system can be displaced with respect to the coil system in the X, Y and Z directions and can be rotated around the X and Y axes. The stroke in the X and Y directions is limited because the coils tightly enclose the objective system in the X-Y plane. When optical record carriers are scanned, a long stroke in the X direction is required so as to be able to place the scanning spot formed by the objective system on each track of the record carrier. To this end the coil system in the known device is placed in a carriage which is movable in the X direction over the required long stroke. A coarse control of the position along the X axis of the objective system can then be realised by means of the carriage, while the system of coils with the ring magnet ensures the fine control along the X axis.
A drawback of the known device is the relatively large mass of the carriage. This makes it difficult to realise a short access time to the tracks on the record carrier.